Organization of centromeric domains in hepatocyte nuclei: rearrangement associated with de novo activation of the vitellogenin gene family in Xenopus laevis.

The existence of a function-dependent, nonrandom organization of chromatin domains within interphase nuclei is supported by evidence which suggests that specific chromatin domains undergo spatial rearrangement under conditions which alter gene expression. Exposure to estrogen of male Xenopus laevis hepatocytes in vitro results in de novo activation of vitellogenin mRNA production and vitellogenin protein synthesis and provides an ideal model to study the association between chromatin organization and changes in gene expression. In a test of the hypothesis that the de novo induction of vitellogenesis in male X. laevis is associated with a spatial rearrangement of specific chromatin domains, centromeric regions were localized by immunofluorescent labeling of associated kinetochore proteins in naive and in estrogen-treated, vitellogenic cells. Analyses by confocal scanning laser microscopy of the three-dimensional spatial distribution of kinetochores in estrogen-treated male hepatocytes showed that a significantly greater proportion of signals was associated with the nuclear periphery than in non-estrogen-treated, naive male cells. In hepatocyte nuclei, quantification of kinetochore signal sizes using image analysis showed that these signals were fewer in number and showed greater variation in size than those of cells in metaphase, with larger signals exhibiting total normalized fluorescence intensities of two, three, four, and five times that associated with kinetochore signals of metaphase cells. These observations are taken to reflect the existence of clustering of kinetochores and, by extension, of centromeres in these cells. In summary, the results show that centromeric domains within interphase nuclei of Xenopus hepatocytes occur as clusters and that these domains undergo spatial rearrangement under conditions which alter the transcriptional state of the cell.